The present invention is concerned with reducing the distortion on one or more modulated signals (also called modulated carriers) that have been transmitted over a non-linear channel. An example of such a channel is a satellite communication channel.
In a satellite communication system the ground station amplifier and/or satellite amplifier are typically operated close to their saturation points, in order to optimally exploit the available power resources. The amplifier, operated in its non-linear region, distorts the signal (this type of distortion is referred to as self-distortion) and causes frequency regrowth, i.e., it increases the bandwidth of the signal after the amplifier beyond the bandwidth occupied by the signal before the amplifier.
Further, in order to optimally exploit the available bandwidth resources, different modulated carriers are typically placed close to each other in the frequency domain, with little or no frequency spacing between the carriers (referred to as guard bands). Due to the non-linear nature of the channel, these carriers interfere with each other. The distortion caused by this interference is referred to as intermodulation.
Summarizing, a modulated carrier suffers from two forms of distortion: self-distortion (distortion of its own signal) and intermodulation (interference from other modulated carriers), due to non-linear amplification of its own signal and of other modulated carriers, respectively. When modulated carriers suffer from self-distortion and intermodulation, the reliability of the communication will be reduced.
A typical satellite communication configuration is shown in FIG. 1. The satellite communication system 100 includes a transmit segment 110, a satellite segment 130 and a receive segment 160. The transmit segment comprises one or more transmitters, typically called modulators. The modulators in the transmit segment 110 can be collocated, but are often geographically separated. A modulator 111 transmits a modulated carrier 121 towards a satellite. The modulated carrier exhibits a certain bandwidth by carrying a modulated signal, which is typically protected by an error-correcting code. A transmission format can for example be, but is not limited to, DVB-S, DVB-S2 or DVB-S2X. Other carriers, transmitted by other modulators are typically placed next to the carrier 121.
A typical uplink spectrum is shown in 120. A person skilled in the art will understand that it is also possible to transmit carriers with partially or completely overlapping bandwidths. For instance, in a bidirectional link one can put the forward and return carrier in the same bandwidth. In this example the receiver can then subtract its own (known) carrier from the received signal, before demodulating the desired (unknown) carrier.
The uplink signal 120 arrives at the satellite input stage. The satellite contains one or more transponders 130. A transponder contains an input multiplexing (IMUX) filter 131, a travelling waveform amplifier (TWTA) 132 and an output multiplexing (OMUX) filter 133. A transponder contains other components as well, such as up- and down-converters, but these are less relevant to the present invention and therefore omitted in FIG. 1.
Due to the IMUX filter 131, only a limited number of carriers is passed on to the input of the TWTA 132. Hence, a TWTA in a transponder has at its input only part of the complete uplink spectrum. In FIG. 1 carrier 121 passes through the IMUX and appears as carrier 141 in the IMUX output signal 140. The TWTA 132 is typically operated close to its saturation point. As a result of the non-linear behaviour in the TWTA, the TWTA output spectrum 142 is affected by in-band and out-of-band distortion. The latter is referred to as spectral regrowth, which increases the original bandwidth 140. The former is referred to as self-distortion. The OMUX filter 133 removes the spectral components that exceed the bandwidth allocated to the corresponding transponder, such that the output signal spectra of the different transponders are non-overlapping prior to recombination 134. Carrier 141 passes through the OMUX and appears as carrier 151 in the downlink spectrum 150. The useful signal in 151 is affected by self-distortion and intermodulation from neighbouring carriers within the spectrum allocated to its corresponding transponder.
As illustrated in FIG. 1, a state-of-art receiver 160 can be used to receive one particular carrier present in the signal 150. This receiver includes a down-converter 161, a matched filter 162, which filters out a single carrier (e.g. carrier 151) and produce a series of received data symbols. Finally, a decoder 163 aims to retrieve the transmitted information bits from the received data symbols. A person skilled in the art understands that proper synchronization is required prior to decoding, however, for the sake of simplicity this is not shown in the drawing. A state-of-art receiver typically treats the self-distortion and intermodulation as additional noise.
There are a number of approaches possible to control and reduce self-distortion and intermodulation in a satellite communication system. One possibility is to apply predistortion to one or more modulated carriers prior to their transmission, with the purpose of anticipating and reducing the distortion incurred by the carriers from the non-linear channel. Several solutions have been presented in the past that apply signal manipulations in the transmit segment in line with this technique. However, this technique suffers from a number of drawbacks which are listed below. The first drawback is that knowledge of the channel characteristics at the transmitter side is required or must be learned. This knowledge can be absent or inaccurate, reducing the effectiveness of the predistortion. A second drawback is that in case multiple carriers are fed through the same non-linear channel, these carriers must be jointly predistorted. This can only be done if these carriers are transmitted from the same geographical location. However, even in that case, jointly predistorting multiple carriers is known to be a computationally intensive operation.
In another approach self-distortion and intermodulation is reduced by applying a corrective action in the receive segment. Few prior art techniques adopt this strategy. U.S. Pat. No. 7,263,144 and U.S. Pat. No. 7,242,725 describe how to compensate distortion of a single carrier, transmitted over a non-linear channel. The invention disclosed in U.S. Pat. No. 8,331,511 presents a joint non-linear interference cancellation method for multi-carrier transmission. The method allows jointly demodulating and decoding a plurality of carriers, whereby the adjacent channel interference is cancelled between neighboring channels. This approach has several drawbacks. The first drawback is that corrective action is needed for all carriers. For certain scenarios one is only interested in the data of one particular carrier. Furthermore, the modulation parameters employed by the other carriers are not necessarily known to the receiver. A second drawback is that the complexity of this receiver is relatively high. This can be partly mitigated by only canceling interference from directly adjacent carriers. However, this obviously introduces a performance penalty.
Hence, there is a need for a solution with low complexity that allows reducing non-linear distortion at the receiver side.